Странно, вроде ошибок нет, у меня работает:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
<script type="text/javascript">
window.onfocus = function() {
document.title += " focus ";
};
window.onblur = function() {
document.title += " blur ";
};
</script>
</head>
<body>
</body>
</html>